JP2003262692A - Boiling water reactor fuel assembly and determination method for fuel arrangement in the fuel assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a boiling water reactor fuel assembly with improved critical power to boiling transition, without disturbing raising enrichment of fuel assembly having partial length fuel rods, and without leading to adverse operation characteristics concerning the nuclear-thermal hydraulics characteristics. <P>SOLUTION: In all fuel rods with high enrichment which does not include burnable poison in at least the cross section positioning in the upper region of the fuel assembly, either of the fuel rods including burnable poison or vacancy region in the cross section neighbors at least either the vertical or the horizontal directions. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a fuel assembly used in a boiling water reactor.

[0002]

2. Description of the Related Art In the field of designing fuel assemblies in boiling water reactors, the average enrichment of fissionable materials represented by uranium 235 is increased and the reactivity is increased to increase the average extraction burnup. The development of a more economical fuel assembly that can be carried out is underway.

When the average enrichment of the fuel assembly is increased, the thermal operation margin such as the maximum linear power density and the minimum limit power ratio becomes small due to an increase in reactivity mismatch during operation, and at the time of low temperature. Causes a problem that the reactor shutdown margin becomes smaller due to the increase in the reactivity of the fuel assembly.

Furthermore, if the average enrichment of the fuel assembly is increased, the neutron spectrum becomes harder, and as a result, the void reactivity coefficient becomes larger on the negative side. For this reason, during operation, the reactivity of the upper part of the fuel with a high void ratio is lower than that of the lower part of the fuel with a low void ratio, so that the axial output distribution tends to distort to the lower part as the enrichment increases. . The increase in the power under the fuel is accompanied by an increase in the maximum linear power density, and the thermal operation margin is reduced.

On the other hand, the large-diameter water rod occupying a region corresponding to a plurality of fuel rods forms a non-boiling region having a large area, so that the axial change of the water-uranium ratio in the axial direction becomes small and the axial output The distribution can be flattened, and as a result, an increase in maximum line power density can be suppressed.

Further, in the fuel assembly, the relative output of the fuel rods in the radial direction of the fuel assembly (fuel rod local peaking) is provided by providing each fuel rod with a dense distribution so as to secure a thermal operation margin. Designed to not be larger than necessary.

On the other hand, when the nuclear fuel material is uranium, the handleable enrichment is 5 wt% or less from the viewpoint of criticality safety in the manufacturing process. Therefore, as the concentration is increased, the distribution of the concentration naturally has a limit in reducing the local peaking of the fuel rods. Further, even if there is no upper limit to the enrichment that can be handled, the enrichment distribution of the fuel rods becomes complicated and the manufacturing cost is increased.

By the way, a fuel assembly having a fuel rod lattice array of 9 rows 9 columns to 10 rows 10 columns (referred to as 9 × 9 fuel and 10 × 10 fuel, respectively) is more fuel rod than the preceding 8 × 8 fuel. The number is 20% to 40% or more. For this reason, the heat load per fuel rod is reduced, and the thermal operation margin is increased, and the fuel rod local peaking can be increased correspondingly.

However, the increase in the number of fuel rods tends to increase the friction pressure loss, and the channel hydraulic stability deteriorates.
Therefore, in the fuel rods used for 9 × 9 fuel and 10 × 10 fuel, in addition to the standard fuel rod having an effective length equal to the effective heat generation length of the fuel assembly, a partial length having a shorter active fuel length than the standard fuel rod is used. There is a conventional technique that employs a plurality of fuel rods as needed. The adoption of the partial length fuel rod helps to reduce the two-phase pressure drop in the upper part of the active fuel length and suppress the deterioration of the channel hydraulic stability.

[0010]

By the way, since the partial length fuel rod has a shorter fuel rod than the other standard fuel rods, the upper cross section of the assembly has a portion (void) where the fuel rod is removed. Will be. During power output operation of the reactor,
The gas-liquid mixed flow flows from the lower part to the upper part, but since the partial length fuel rod replaces the void part in the middle, a new lateral flow of fluid is generated on the upper part side. Since the resistance in the voids decreases, the coolant flow rate in the voids increases, while the flow rates in other regions tend to decrease. This affects the likelihood of boiling transitions in the fuel rods. The boiling transition is likely to occur on the upper side of the fuel assembly. In general, the fuel rods near the voids have more coolant than other regions, so the limit output to the boiling transition is improved, while in other regions, the coolant output becomes insufficient and the limit output tends to decrease. To have.

Thus, if the limit output of the fuel rod whose output tends to increase can be improved, the limit output of the fuel assembly can be improved.

The present invention allows a fuel assembly having a partial length fuel rod to undergo boiling transition without impairing the enrichment and without deteriorating the operating characteristics related to nuclear thermal-hydraulic characteristics. An object of the present invention is to obtain a fuel assembly for a boiling water nuclear reactor, which can improve the limit output.

[0013]

The fuel assembly for a boiling water nuclear reactor according to the invention described in claim 1 comprises a fuel rod group in which a fuel pellet made of an oxide of a nuclear fuel material is filled in a fuel cladding tube. A fuel assembly for a boiling water reactor, which is regularly arranged in a square lattice array of 9 rows and 9 columns or more and has large diameter water rods occupying an area corresponding to the plurality of fuel rods, wherein the fuel rods are A standard fuel rod having a fuel active length equal to the fuel active length of the fuel assembly, and a partial length fuel rod having a void active region having a shorter fuel active length than the standard fuel rod and lacking the fuel rod in the upper region. In all fuel rods having the highest enrichment containing no burnable poison in at least a partial cross section located in the upper region of the fuel assembly, the burnable poison-containing fuel rod in the cross section is Or the sky One of regions are those that are adjacent in at least one direction of the longitudinal or transverse direction.

In the boiling water reactor fuel assembly according to the second aspect of the present invention, all the fuel rods containing no combustible poison in the cross section located in the upper region of the first aspect are , The highest concentration except for the corners of the aggregate and the vicinity of the corners.

In the fuel assembly for a boiling water reactor according to the third aspect of the present invention, at least half of the partial length fuel rods according to the first or second aspect are at the outermost periphery or have the large diameter. It is located adjacent to the water rod.

According to the invention as set forth in claim 4, the method for determining the fuel rod arrangement in the fuel assembly for a boiling water reactor is the fuel rod arrangement in the fuel assembly as set forth in any one of claims 1 to 3. In the method of determining, the maximum enrichment fuel rods are temporarily arranged except for some of the fuel rods at the corners of the assembly and the outermost peripheral fuel rods in the vicinity of the corner fuel rods, and all the highest enrichment fuel rods have the above-mentioned cross section. Of at least one burnable poison-bearing fuel rod or part-length fuel rod adjacent to the burnable poison-bearing fuel rod in at least one of the longitudinal or horizontal directions in the above Is.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, attention was paid to the cross section located in the upper region of the fuel assembly where the boiling transition occurs. The likelihood of boiling transition of the fuel rod depends on the output of the fuel rod in addition to the influence of the partial length fuel rod as described above.
That is, it was noted that the boiling transition is less likely to occur as the output of the fuel rod itself is smaller and the output of the adjacent fuel rod is smaller.

Specifically, in the present invention, all the fuel rods having the highest enrichment without combustible poisons in the cross section located in the upper region of the fuel assembly contain the combustible poisons in the cross section. Either the fuel rods or the void regions are adjacent in at least one of the longitudinal or lateral directions. As a result, it is possible to achieve an increase in the limit output up to the boiling transition without impeding the increase in the degree of enrichment and without deteriorating the operating characteristics related to the nuclear thermal hydraulic characteristics.

That is, in order to increase the enrichment, it is necessary to use a large number of fuel rods having the highest enrichment even in the upper region.
If there is a constraint of less than%, it is unavoidable to use more fuel rods with the highest enrichment. In this case, the fuel rod output of the highest enrichment fuel rod is increased, and there is a concern that the limit output leading to boiling transition may be reduced.

In order to avoid this, the fuel rod having a small output and the void region may be adjacent to the highest enrichment fuel rod. Therefore, in the fuel assembly of the present invention, in the upper region, all the highest enrichment fuel rods containing no combustible poison are
Adjacent to either the fuel rod containing the burnable poison in at least one of the longitudinal or lateral directions, or the void area.

As a result, the limit output of the highest enrichment fuel rod is improved, and thus the limit output characteristic of the fuel assembly is improved. If a fuel rod with a concentration less than the maximum enrichment is arranged adjacent to the fuel rod with the highest enrichment, the neutron spectrum around the fuel rod will tend to be softened, and as a result, the output of the fuel rod with the highest enrichment will increase rather. Because it ends up, the expected effect cannot be obtained.

Generally, from the viewpoint of economic efficiency, it is considered desirable to add the burnable poison to the extent that the neutron absorption effect is lost at the end of the cycle. On the other hand, from the viewpoint of safety, it is necessary to secure a sufficient stop margin even at the end of the cycle. Therefore, particularly in the upper region of the fuel assembly that greatly contributes to the stop margin, it is necessary to add the burnable poison in advance to the extent that the neutron absorption effect does not completely disappear even at the end of the cycle. As described above, since the neutron absorption effect remains in practice even at the end of the cycle, the fuel assembly of the present invention operates when it is loaded into the core as an unburned fuel assembly, that is, a new fuel. The limit output can be improved from the beginning of the cycle to the end. It should be noted that the limit output of the fuel assembly during operation becomes small from the early stage to the middle stage of the operation cycle where the distortion of the power distribution in the core becomes large as the excess reactivity becomes high and the amount of control rods inserted becomes large.

Further, in the present invention, preferably, in the cross section located in the upper region, all the fuel rods containing no combustible poisons have the highest concentration except the corners of the assembly and the outermost peripheral positions near the corners. Distribute the fuel rod of the degree. This can contribute to a higher degree of concentration.

Further, a portion of the partial length fuel rod, preferably
Placing at least half of the fuel rods on the outermost periphery or adjacent to the large diameter water rods has an advantage that the number of fuel rods having a concentration less than the maximum enrichment and fuel rods containing burnable poisons can be small.

[0025]

(Embodiment 1) FIG. 1 is an explanatory view showing a cross-sectional structure of an embodiment of a fuel assembly of the present invention. With a 9 × 9 fuel structure, a water rod area for seven fuel rods is secured in the central portion. Two large-diameter water rods can be arranged here, for example. Each square corresponds to a fuel rod. The numbers indicate the levels of enrichment, indicating that the enrichment is higher in the order of 1, 2, 3 ... That is, 1 is the highest enrichment. In the figure, the enrichment number 1 is hatched. G is a fuel rod containing gadolinia which is a burnable poison. Further, P is a partial length fuel rod, and this portion becomes a void in the upper region of interest in the present invention. In this example, eight partial length fuel rods are arranged in the second layer.

As can be seen from FIG. 1, the highest enrichment fuel rods are adjacent to the gadolinia-containing fuel rods or the partial length fuel rods in at least one longitudinal or transverse direction. This improves the critical power characteristics of all highest enrichment fuel rods. In addition, corner four corners (1a and symmetrical position),
The outermost peripheral fuel rods (2a and symmetrical positions) adjacent to this are positions where the output is likely to increase, so
Fuel rods below maximum enrichment are placed. Further, with respect to the outermost fuel rods (3a and symmetrical positions) located next to it, the enrichment is reduced and the output of the fuel rod itself is reduced to prevent the deterioration of the limit output.

(Embodiment 2) FIG. 2 is an explanatory view showing the cross-sectional structure of another embodiment of the fuel assembly of the present invention. This example is also 9
With a × 9 fuel structure, fuel rods 7 are provided in the central portion as in the first embodiment
Although it has water rod regions for the number of rods, the number and arrangement of the partial length fuel rods are different. Part length fuel rod is 4 at the center of each side of the outermost circumference.
Book, two were placed adjacent to the water rod. Arranging the partial length fuel rods on the outermost circumference can be expected to improve the stop margin and alleviate the void coefficient more than when the partial length fuel rods are arranged around the water rod. As in Example 1, all the highest enrichment fuel rods are adjacent to the gadolinia-filled fuel rods or the partial length fuel rods in at least one longitudinal or transverse direction.

(Embodiment 3) FIG. 3 is an explanatory view showing a cross sectional structure of still another embodiment of the fuel assembly of the present invention. This example has a 10 × 10 fuel structure, and a water rod region for nine fuel rods is secured in the central portion. Here, for example, one water rod having a rectangular shape can be arranged. 8 partial length fuel rods in the second layer, 4 adjacent to the water rods
The book is arranged. In the case of 10 × 10 fuel, since the thermal margin increases as the surface area of the fuel rod increases, there is no problem even if the relative output of the fuel rod is increased to some extent. Therefore, in this embodiment, all the fuel rods not containing gadolinia have the highest enrichment except for the four corners, and emphasis is placed on increasing the average enrichment. On the other hand, many gadolinia-containing fuel rods are arranged in the second layer to prevent the critical output characteristics of the outermost peripheral fuel rods from being excessively deteriorated. As with the previous embodiment, all high enrichment fuel rods are adjacent to gadolinia-filled fuel rods or partial length fuel rods in at least one longitudinal or transverse direction.

(Embodiment 4) FIG. 4 is an explanatory view showing a cross sectional structure of still another embodiment of the fuel assembly of the present invention. This example has a 10 × 10 fuel structure similar to that of Example 3, except that there are 14 partial length fuel rods in total, two on each side of the outermost periphery, two on the corners and adjacent to the water rod. I arranged four of them. With this configuration, it is possible to expect a large improvement in the stop margin and a reduction in the absolute value of the void coefficient, which only compensates for the high enrichment.

The arrangement of the fuel rods with gadolinia was determined by the following method. First, the highest enrichment fuel rods are arranged except for the corner fuel rods, and all the highest enrichment fuel rods are adjacent to at least one gadolinia-containing fuel rod or partial length fuel rod in at least one longitudinal or lateral direction. As described above, a part of the highest enrichment fuel rod is replaced with a fuel rod containing gadolinia. Although the same applies to the previous embodiment, the present invention can be configured to satisfy a predetermined condition by such an extremely simple method. Needless to say, this method can be similarly applied to the first to third embodiments.

As described above, according to the present invention, the output is high and the boiling transition is suppressed without sacrificing the enrichment and other operating characteristics and without complicating the fuel rod arrangement. Since it is possible to improve the limit output of the fuel rod with the highest enrichment that is likely to occur, and eventually improve the limit output characteristics of the fuel assembly, a fuel assembly suitable for high burnup with excellent thermal operation margin. Can provide the body.

[0032]

As described above, the present invention does not impair the enrichment of the fuel assembly having the partial length fuel rods and causes deterioration of the operating characteristics relating to the nuclear thermal hydraulic characteristics. It is possible to obtain a boiling water reactor fuel assembly that improves the limit output up to the boiling transition. In the above description, uranium was taken as an example of the nuclear fuel substance, but the present invention does not lose its effect even when applied to other combustible poisons and nuclear fuel substances. Other nuclear fuel materials may include plutonium, but in this case, the uranium enrichment need only be read as plutonium enrichment.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a cross-sectional configuration of an embodiment of a fuel assembly of the present invention.

FIG. 2 is an explanatory view showing a cross-sectional structure of another embodiment of the fuel assembly of the present invention.

FIG. 3 is an explanatory view showing a cross-sectional configuration of still another embodiment of the fuel assembly of the present invention.

FIG. 4 is an explanatory view showing a cross-sectional configuration of still another embodiment of the fuel assembly of the present invention.

[Explanation of symbols]

1 ... Highest enrichment fuel rod, 2 ... the fuel rod with the second highest concentration, 3 ... the fuel rod with the third highest concentration, 4 ... the fourth highest fuel rod enrichment, G ... Fuel rod with gadolinia, P ... Partial length fuel rod,

Claims (4)

[Claims] 1. A fuel rod group in which a fuel pellet made of an oxide of a nuclear fuel material is filled in a fuel cladding tube is regularly arranged in a square lattice array of 9 rows and 9 columns, and an area corresponding to the plurality of fuel rods is formed. A fuel assembly for a boiling water reactor, comprising a large diameter water rod occupying, wherein the fuel rod is a standard fuel rod having a fuel active length equal to the fuel active length of the fuel assembly, and a standard fuel rod Also has a short active fuel length and has a partial length fuel rod having a void region where the fuel rod is missing in the upper region, and contains burnable poison in at least a part of the cross section located in the upper region of the fuel assembly. Not all fuel rods with the highest enrichment are characterized in that in the cross-section either the burnable poison-bearing fuel rod or the void region is adjacent in at least one longitudinal or lateral direction. When Boiling water reactor fuel assembly that. 2. The cross-section located in the upper region is characterized in that all fuel rods which do not contain burnable poison have the highest enrichment except at and near the corners of the assembly. The fuel assembly for a boiling water reactor described. 3. The boiling water reactor according to claim 1, wherein at least half of the partial length fuel rods are located at the outermost periphery or at a position adjacent to the large diameter water rod. Fuel assembly. 4. The method for determining the fuel rod arrangement in a fuel assembly according to any one of claims 1 to 3, wherein the highest enrichment fuel rod is temporarily arranged except for the corners of the assembly and the fuel rods near the corners. However, all the highest enrichment fuel rods should be adjacent to at least one burnable poison-bearing fuel rod or partial length fuel rod in at least one longitudinal or transverse direction in the cross section. A method for determining the arrangement of fuel rods in a fuel assembly for a boiling water reactor, characterized in that a part of the fuel rods is replaced with burnable poison-containing fuel rods.